Foamed thermoplastic polymers enjoy widespread use in building construction due to their low density and excellent heat insulating properties. For such uses, it is desirable that the foam be flame resistant and self-extinguishing in order to avoid any potential fire hazard. A variety of halogenated organic compounds, including hexabromocyclododecane, have been proposed as additives for this purpose.
Hexabromocyclododecane has been incorporated into foamed thermoplastics by several different methods. For example, Jpn. Kokai 63-215740 teaches that polystyrene and hexabromocyclododecane may be dry blended and then extruded with a volatile foaming agent. This method is not suitable for the preparation of expandable polystyrene beads, however.
U.S. Pat. No. 4,761,432 teaches a method of coating seed beads of polystyrene with hexabromocyclododecane wherein emulsion polymerization of a vinyl aromatic monomer is carried out in the presence of the fire-retardant and the seed beads. Incorporation of the fire-retardant was to be much more effective using the coating method than when the fire-retardant was added with the foaming agent during impregnation (Example II).
Another method of preparing expandable polystyrene beads containing hexabromocyclododecane is to polymerize styrene monomer in an aqueous suspension containing foaming agent and the fire-retardant, as illustrated by U.S. Pat. Nos. 3,093,599, 3,956,203, 3,503,905 and 4,281,067. While such procedures are effective and require a minimum number of processing steps, the presence of the hexabromocyclododecane during polymerization may possibly result in undesired changes in the molecular weight of the polystyrene produced due to chain termination or transfer reactions.
U.S. Pat. No. 3,093,599 teaches that polystyrene beads preimpregnated with a volatile hydrocarbon foaming agent may be rendered fire-retardant by coating the beads with hexabromocyclododecane using an adhesive agent such as paraffin wax. Such methods require a separate impregnation step and do not yield beads having the hexabromocyclododecane physically incorporated within the polymer. In addition, the coating may interfere with the desired expansion and fusion of the beads upon molding.
U.S. Pat. No. 4,520,136 describes an impregnation method whereby polystyrene beads and hexabromocyclododecane are suspended in water and heated to incorporate a volatile hydrocarbon blowing agent. Unless pentaerythritol tetrastearate is also present, however, foams prepared from the resulting impregnated beads have poor flame resistance.
It is clear there is a need for an improved method of incorporating hexabromocyclododecane into expandable vinyl aromatic polymer beads whereby molded foam articles prepared from the impregnated beads may be effectively rendered flame resistant.